Metal detecting apparatus for conveyor belt

ABSTRACT

For detecting metal traveling along a conveyor belt, an excitation coil and a pair of pickup coils are positioned adjacent the belt. A signal voltage is induced in the pickup coils whenever a metallic body passes by such coils. Direct pickup from the excitation coil by the pickup coils is reduced to a minimum by means of a bucking voltage which is automatically adjusted in phase and amplitude. A separate detecting means automatically reduces the sensitivity of the indicating means whenever a metallic nonuniformity in the belt itself passes by the pickup coils.

United States Patent Inventors METAL DETECTING APPARATUS FOR CONVEYORBELT Primary Examiner-Donald J. Yusko Assistant ExaminerPerry PalanAttorneys-George L. Church, Donald R. Johnson, Wilmer E.

McCorquodale, Jr. and Frank A. Rechif 15 Claims, 8 Drawing Figs.

U.S. For detecting metal traveling along a conveyor 324/4L340/259 belt,an excitation coil and a pair of pickup coils are posi- 1nt.Cl ..G08b21/00, i d dj t the belt. A signal voltage is induced in the 33/ 12pickup coils whenever a metallic body passes by such coils. Field ofSearch 340/258, Direct pickup f the excitation Co by the pickup coils is258 258 (C), 259; 324/38 (L), 34 reduced to a minimum by means ofabucking voltage which is R f d automatically adjusted in phase andamplitude. A separate dee erences tecting means automatically reducesthe sensitivity of the in- UNITED STATES PATENTS dicating means whenevera metallic nonuniformity in the belt 2,943,306 6/1960 Gray et al.340/258 itself passes by the pickup coils.

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I SHEEII'OFS' CRIT\CAL- COUPLING Ficj f AMOUNT or OVERLAP HENRY L.BACHOFER Z ELBERT N. HAWHAN FIGS.

BELT TRAVEL INVENTORSZ HENRY L.BACHOFER 5/ ELBERT N. H HAN ATTY.

PATENTED APR 6197i SHEET 5 [IF 5 INVENTORSI Fll AM 305.20 A w HENRY L.BACHOF' ER g Tug 0? Km ELBERT N SHAWHAN ATTY.

METAL DETECTING APPARATUS FOR CONVEYOR BELT This invention relates tometal detecting apparatus, and more particularly to apparatus fordetecting the presence of tramp metal (metal fortuitously present) on aconveyor belt.

In the northern region of the Province of Alberta, Canada, an orebody ofso-called tar sand is being mined and transported to a processing plantfor extraction of a valuable petroleum material therefrom. The mining isdone by the utilization of a large bucketwheel, and the mined materialis dumped onto a wide and long conveyor belt for transportation of suchmaterial to the processing plant.

It is known that metal drill casing was left in the orebody now beingmined, after previous attempts to produce oil from the tar sand. Thelocation of this drill casing is not known. Jagged pieces of this pipeor casing, torn out by the bucketwheel, could cause serious damage tothe long sections of the con conveyor belt, since they would tend tocause lengthwise rips that could not be repaired. It is thereforeimportant to detect metallic bodies, which may be fortuitously resent onthe conveyor belt, before they can seriously damage such belt.

Therefore, an object of this invention is to provide a novel metaldetecting apparatus.

Another object is toprovide a novel apparatus for detecting the presenceof a metallic body traveling at a known rate through a detection zone.

A further object is to provide a novel apparatus for detecting thepresence of tramp metal on a conveyor belt.

In order to provide the necessary strength, the conveyor belt isreinforced with steel cables which extend throughout its length.Furthermore, at intervals in the belt there are splices and patches, atwhich locations the reinforcing cables are spaced at double density. Inaddition to this, large masses of metal (e.g., metallic parts, tools,etc.) are moved near the belt during nomial operation. Moreover, it isdesired to detect either ferrous or nonferrous metal on the belt. Also,the high rate of movement of mined material on the belt (for example 3million tons per month, or considerably one 1 ton per second), togetherwith the presence of rock, etc., causes a considerable amount ofvibration and shock to be set up.

Accordingly, still another object of the invention is to provide a metaldetecting apparatus which will detect the presence of only tramp metalon a conveyor belt, in the presence of reinforcing metal contained inthe belt itself, and in the presence of metal which may be moving in thevicinity of the belt but which is not on the belt.

A still further object is to provide novel metal detecting apparatus fora conveyor belt which will detect either a ferrous or a nonferrous metalbody on the belt.

Yet another object is to provide novel metal detecting apparatus for aconveyor belt wherein the coil structure, which is positioned adjacentthe belt, is of extremely rigid constructron.

The foregoing and other objects of the invention are accomplished,briefly, in the following manner: A rigid one-turn excitation coil and apair of rigid one-turn pickup coils are mounted above a conveyor belt,with the pickup coils located on respective opposite sides of thecritical coupling position with respect to the excitation coil. Directpickup from the excitation coil by the pickup coils is reduced to aminimum by means of a bucking voltage which is automatically adjusted inphase and amplitude through the operation of a self-balancing system,the time constant of this system being in excess of the time requiredfor a metallic body on the conveyor belt to move past the pickup coils.An indicating (alarm) circuit is operated by the signal voltage outputof the pickup coils. A separate coil, located below the belt, produces avoltage in response to the passage of a belt patch or splice past suchcoil, and this voltage is utilized to reduce the sensitivity of theindicating circuit during the passage of this belt or splice past thepickup coils.

A detailed description of the invention follows, taken in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a diagrammatic illustration of three overlapping one-turncoils utilized in the invention;

FIG. 2 is a graph representing the variation of magnetic flux withoverlap of two coils;

FIGS. 3-5 are views illustrating the belt-loop arrangement utilized inthe invention;

FIG. 6 is a combined detailed and block diagram of the circuitryutilized in the invention;

FIG. 7 is a schematic diagram of the self-balancing circuit of theinvention; and

FIG. 8 is a schematic diagram of the alarm and belt splice compensatorcircuits of the invention.

Refer first to FIG. 1. The sensing portion of the apparatus (i.e., theportion of the apparatus which is associated with the conveyor belt)comprises three coils, an excitation coil 4 and two pickup coils 5 and6. Each of these coils is in the form of a rectangular one-turn loopformed from four lengths of threeinch aluminum l-beam, welded or rigidlybolted at the corners to form a rectangle typically 3 feet by 4 fleet insize. This construction provides rigid coils of ample size, highlyresistant to vibration and shock.

The loops described are coupled to the external circuitry to bedescribed hereinafter by means of individual toroidal coils, each coilbeing wound on a toroidal core which surrounds an I-beam of eachrespective rectangle. The excitation coil 3 is wound on a toroidal corewhich surrounds one l-beam of loop 4, one pickup coil 7 is wound on atoroidal core which surrounds one I-beam of loop 5, and the other pickupcoil 8 is wound on a toroidal core which surrounds one l-beam of loop 6.

The ends of coil 3 are connected to the high-level output of anamplifier which is driven by a fixed-frequency source such as a crystaloscillator, thereby to supply oscillatory energy of fixed frequency tothe'loop 4. The pickup coils 7 and 8 are connected in series-aidingrelation with reference to an external flux passing through the loops 5and 6, and the remaining two ends of these coils are connected to supplyan input signal to the circuitry illustrated in FIG. 6. The pickup loops5 and 6 overlap the excitation loop 4 to a certain extent, but thepickup loops are placed on respective opposite sides of the criticalcoupling position (indicated by the dotted line A) with respect to theexcitation coil.

Now refer to FIG. 2, which is a graph representing the amplitude andrelative sense of the net flux (from the excitation loop) passingthrough a pickup loop, versus amount of overlap. At the criticalcoupling position, there is zero net flux through the pickup loop; oneither side of this critical coupling position, the net fluxes areopposite in sense. Since in FIG. 1 the pickup loops 5 and 6 are onrespective opposite sides of the critical coupling position A, the netfluxes (from the excitation loop 4) through the two pickup loops areopposite in sense. Therefore, the direct pickup from the excitation loop4 tends to cancel in the pickup loops 5 and 6; however, the effects ofan external flux through the pickup loops add, due to the series-aidingconnection of the coils 7 and 8.

FIGS. 3, 4, and 5 are somewhat diagrammatic views illustrating the threecoils or loops previously described as they would be associated with aconveyor belt, to detect metallic bodies traveling along such belt; FIG.3 is a diagrammatic side elevation, FIG. 4 is an end elevation of thebelt-loop arrangement, and FIG. 5 is a top or plan view of thearrangement.

FIG. 3 illustrates diagrammatically the loops 4, 5, and 6 operativelyrelated to the conveyor belt 77, which is shown in side elevation. Thedirection of travel of the belt is as indicated. The loops 4, 5, and 6are all located in horizontal planes at a distance of about 3 feet abovethe belt, with the 3 foot dimension of the loops extending parallel tothe direction of belt travel. As in FIG. 1, the pickup loops 5 and 6overlap the excitation loop 4, being on respective opposite sides of thecritical coupling position A. For simplification, the toroidal couplingcoils for the loops are not illustrated in FIG. 3.

FIGS. 4 and 5 illustrate the manner of supporting the loop assembly inposition above the conveyor belt. The three hollow aluminum rectangles4, 5, and 6 are joined together as an assembly by strips 88 of phenolicresin (e.g., 1 inch thick strips) bolted on the sides of the threeI-beam frames (loops), the bolt holes being slotted to permit accurateadjustment of position of the three rectangular loops. It will beappreciated that the 4 afoot dimension of the rectangular loops extendsin the horizontal direction in FIG. 4. As shown in FIG. 4, the conveyorbelt 77 is somewhat dished, seen in end elevation.

The conveyor belt structure conventionally includes fixed side rails 89.Mounting brackets 90 (shown as four in number, two on each side; seeFIG. are bolted to the tops of the two side rails and to the side plates88, to rigidly support the loop assembly 4, 5, 6 in position above thebelt 77.

Now refer to FIG. 6, which is a combined block and schematic circuitdiagram (simplified) of the metal detecting apparatus of this invention.As previously described, an excitation coil and two pickup coils areused; for simplification of the illustration, however, only one pickupcoil is shown in FIG. 6. As previously stated, the pickup from theexcitation coil tends to cancel in the two pickup coils, while theeffects of an external flux through these two latter coils add. Theresidual direct pickup from the excitation coil is reduced to a minimumlevel by means ofa bucking voltage which is automatically adjusted inphase and amplitude; this bucking voltage is algebraically added to theoutput voltage of the pickup coils, to produce a resultant outputvoltage. The automatic adjustment of the phase and amplitude of thebucking voltage are accomplished by means ofa self-balancing circuit.

The output of-a crystal oscillator 1, operating for example at afrequency of 7.35 kHz. is amplified by a driver amplifier 2 ofconventional type, such as a commercially-available solidstate(all-silicon-transistor) public address (PA) or booster type ofamplifier. A portion of the high-level output ofthis amplifier is fed tothe toroidal coil 3 which surrounds the one turn excitation loop 4;thus, this portion of the amplifier output is used as excitation currentfor coil 4. The pickup loops 5 and 6 (loop 5 being illustrated in FIG.6) overlap the excitation loop 4, as previously described; one end ofthe toroidal coil 7 (in reality, this would be one free end of theseries-connected toroidal coils 7 and 8, which respectively surround thepickup loops 5 and 6) is connected to ground, and the other free end ofthe toroidal coil combination is connected through a resistor 9 to theinput of a band pass filter 10.

A bucking voltage to cancel the unbalance in the pickup coils 5 and 6(which is to say, in the coils 7 and 8, which latter serve mainly ascurrent transformers, coupled to the respective one-turn loops) isgenerated by adding components in each of the four quadrants. Thesequadrature-related voltages are obtained from the driver amplifier 2. Alow-level output is taken from amplifier 2 and fed to the primarywinding 11 of a transformer 12 which has a centertapped secondarywinding 13. The secondary centertap is grounded, and at one end ofwinding 13 the voltage is fed through two paralleled butoppositely-arranged RC networks 14 and 15 (detailed hereinafter) toprovide respective voltages of 0 and 90 phase. The resistor 16 in serieswith the diode 17 forms a voltage divider which determines the amplitudeof the 0 component applied to a combining network 18 for summing withthe other three components; the combining network 18 is located at theinput of an amplifier 19. The resistor 20 in series with the diode 21forms a voltage divider which determines the amplitude of the 90component applied to the combining network 18 for summing with the otherthree components. The diodes 17 and 21 function as voltage-controlledresistors, in a manner which will be described hereinafter.

At the other end of winding 13, the voltage is fed through twoparalleled but oppositely-arranged RC networks 22 and 23 to providerespective voltages of 180 and 270 phase. The resistor 24 in series withthe diode 25 forms a voltage divider which determines the amplitude ofthe l80 component applied to the combining network 18 for summing withthe other three components. The resistor 26 in series with the diode 27forms a voltage divider which determines the amplitude of the 270component applied to the combining network 18 for summing with the otherthree components. The diodes 25 and 27 also function asvoltage-controlled resistors.

The four quadrature-related voltages (derived from amplifier 2, andindividually controlled in amplitude by means of the controlled diodes17, 21, 25, and 27) are summed or combined by means of network 18, andthe resultant is amplified by amplifier 19. The output of amplifier 19(which is the bucking voltage previously referred to, automaticallyadjusted in phase and amplitude as will be described subsequently) isfed through a capacitor 28 and a resistor 29 to the input of filter 10,at which point it is algebraically added to the unbalance voltage(induced in pickup coil 7). The algebraic sum is filtered by filter 10,to remove any harmonics generated in the coils 5 and 6 (or 7 and 8), orany harmonics due to the nonlinearity of the diodes 17, 21, 25, and 27.

The net voltage (output of filter 10) is amplified in an amplifier 30,and is then rectified in two synchronous detectors or rectifiers 31 and32, each of which is analogous to a socalled phase detector. Thesesynchronous rectifiers 31 and 32 are driven by voltages obtained fromthe driver amplifier 2 and displaced in phase from each other. Ahigh-level output is taken from amplifier 2 and fed through twoparalleled but oppositely-arranged RC networks 33 and 34 to providerespective voltages of 0 and 90 phase. The 0 phase voltage drivessynchronous detector 32 and the 90 phase voltage drives synchronousrectifier 31. An output voltage (control voltage), which may be eitherpositive or negative depending upon the sense of the net of resultantvoltage at the input of filter 10, is taken from detector 31 and isapplied through a resistor 35 to the 270 diode 27, and also through aresistor 36 to the 90 diode 21. An output voltage (control voltage),which may be either positive or negative depending upon the sense of thenet or resultant voltage at the input of filter 10, is taken fromdetector 32 and is applied through a resistor 37 to the diode 25, andalso through a resistor 38 to the 0 diode 17.

In operation, for any particular unbalance in the pickup coils 7 and 8(i.e., for any particular resultant voltage'at the input of filter 10),two of the four diodes 17,21, 25, and 27 in the bucking voltageamplitude adjusting circuit become conducting; which two conduct dependson the polarities of the synchronous detector outputs (plus or minuscontrol voltages). Bucking voltage (phase-quadrature) components(derived from the driver amplifier 2, by way of transformer secondary13) corresponding to the conducting diodes are lower in amplitude thanthe other two, and are controlled by the magnitudes of the controlvoltages from the detectors 31 and 32. Equilibrium is established whenthe net or resultant voltage at the input of filter 10 is a minimum.Thus, the phase and amplitude of the summed or resultant bucking voltage(the output of network 18) are automatically adjusted by means of theself-balancing circuit described (including the synchronous detectors 31and 32, the diodes 17, 21, 25 and 27 etc.) in such a way as to reducethe net or resultant output voltage of the pickup coils (i.e., thevoltage at the input of filter 10) to a minimum value.

Refer now to FIG. 7, which is a detailed schematic of the self-balancingcircuit. The quadrature-related components for the bucking voltage aredeveloped by means of the networks 14, 15, 22, and 23, each of whichcomprises the series combination of a resistor and a capacitor. Eachcauses a 45 phase lag across the capacitor, relative to the inputvoltage. The 0 network 14 comprises a resistor 39 in series with acapacitor 40, the voltage across resistor 39 being taken off for thevoltage divider 16, 17. The 90 network 15 comprises a capacitor 41 inseries with a resistor 42, the voltage across capacitor 41 being takenoff for the voltage divider 20, 21. The 180 network 22 comprises aresistor 43 in series with a capacitor 44, the voltage across resistor43 being taken off for the voltage divider 24, 25. The 270 network 23comprises a capacitor 45 in series with a resistor 46, the voltageacross capacitor 45 being taken off for the voltage divider 26,27.

The two networks 33 and 34, for providing the two quadrature-relateddriving voltages for the respective synchronous detectors 32 and 31,each comprise the series combination of a resistor and a capacitor,similarly to networks, 14, 15, 22, and 23 previously described. Thenetwork 33 comprises a resistor 47 in series with a capacitor 48, thevoltage across resistor 47 being taken off for application to thesynchronous detector 32 by way of an input transformer 49. The 90network 34 comprises a capacitor 50 in series with a resistor 51, thevoltage across capacitor 50 being taken off for application to thesynchronous detector 31 by way of an input transformer 52.

The time required for the self-balancing circuit to balance isdetermined by the series resistors 53 and the shunt or storagecapacitors 54 associated with the synchronous detectors 3] and 32. Thistime constant is made about 0.5 second, to allow time for full responseto a metallic body traveling at belt speed past the pickup coils and 6.

When a metallic body momentarily enters the field of the excitation coil4, eddy currents are induced in the metal, thus creating a field andinducing a signal voltage in the pickup coils 7 and 8. If the metal isferrous, there is an additional voltage component, due to distortion ofthe excitation field as a result of the change in permeability.

The time constant of the self-balancing circuit-previously described ischosen so that a piece of metal on the belt, moving at the known speedof the belt (e.g., about 13 feet per second) past the pickup coils 5 and6, causes a net signal in the pickup coils 5 and 6 (or 7 and 8). That isto say, when a metallic body passes the coils at the speed of the belt,the selfbalancing circuit cannot respond in time to cancel (by means ofthe bucking voltage) the unbalance voltage across the pickup coils 7 and8, and a net voltage appears at the input of the band pass filter 10.This voltage is amplified by amplifier 30 and rectified by a rectifier55 which is coupled to the output of amplifier 30.

lf the metallic body remains in the field of the excitation coil for aninterval longer than the time constant of the self-balancing circuit(such a condition arising, for example, as a result of metal moving inthe vicinity of the belt, or as a result of mechanical flexing of thecoils 4, 5, or 6), the resulting signal voltage induced in the pickupcoils is cancelled by the selfbalancing circuit, which automaticallyadjusts the amplitude and phase of the bucking voltage in a mannerappropriate to effect such cancellation.

When a metallic body passes the coils at the speed of the belt, a net(uncancelled) voltage appears at the input of the band pass filter 10,and this voltage is amplified at 30 and rectified at 55 to produce a DCpulse. This DC pulse is amplified further by means of an amplifier 56and is then applied to a Silicon Control Rectifier (SCR) alarm circuit57.

Now refer to FIG. 8, which is a detailed circuit diagram of the alarmcircuit portion of the apparatus, together with other circuits directlyassociated therewith. Rectifier 55 is coupled to the output of amplifier30 by way of a capacitor 58. The DC pulse produced by rectifier 55 ispassed through a broad band pass filter which rejects signals muchslower or faster than a signal from a metallic body moving at belt speed(the fast signals might arise, for example, asv a result of vibration orshock). The series resistor 59 and the bypass capacitor 60 set the upperfrequency limit of the broad band pass filter referred to. The largeseries capacitor 61 and the resistance of the sensitivity controlpotentiometer 62 set the lower frequency limit of the broad band passfilter. The sensitivity control potentiometer 62 adjusts the level ofthe pulse required to fire the SCR 63, after further amplification inamplifier 56.

The coil 64 of a relay 65 is normally energized (thereby picking up thisrelay) by the flow of current from a positive DC source through a seriesresistor 66 and a pair of latching contacts 67 which are closed when therelay picks up. In this connection, it is pointed out that the negativeterminal of the DC source referred to is grounded. Relay 65 is thereforeillustrated in the energized or picked-up condition. When the level ofthe signal (amplified DC pulse) appearing at the output of amplifier 56is sufficient to overcome the bias applied through a resistor 68 to theSCR 63 (one means for applying such a bias will be describedhereinafter), the SCR is fired. This causes a flow of current from theDC source, through resistor 66, a pair of relay contacts 69 which areclosed when the relay is picked up, and the anode-cathode path of theSCR 63 to ground. The increased current flow through resistor 66 thencauses the voltage across the relay coil 64 to decrease, and relay 65drops out, that is, becomes deenergized. The dropping out of the relay65 opens its contacts 69, which breaks the circuit to the SCR 63; theSCR is then ready to be tired again.

After the SCR 63 has been fired, the alarm circuit 57 is not armed untilthe manual reset button 70 is pressed, to establish an energizationcircuit through the relay coil 64. This energizes or picks up the relay65, reclosing its latching contacts 67 (which thereby hold the relaypicked up), and reclosing the contacts 69, which reestablish theconnection to the SCR 63.

When the relay 65 drops out, a pair of contacts 71 are closed. Thiscompletes a circuit from the AC power line through a counter coil 72, aneon lamp 73, and a diode rectifier 74; the counter is then advancedonestep and the lamp 73 is lighted. When the reset button 70 is pressed,relay 65 is picked up, opening the contacts 71 and extinguishing thelamp 73. This indicates that the circuit is properly armed for the nextsignal.

The remaining set of relay contacts (comprising a pair of contacts 75which are closed when the relay 65 is picked up, and a pair of contacts76 which are closed when the relay drops out) serve as alarm contacts,which can be used to operate an external alarm or to operate a contactorwhich stops the belt. The overall response time of the apparatus of thisinvention, to a metallic body traveling at belt speed past the pickuploops 5 and 6, is such that the body is detected, and an alarm given,before any irremediable damage to the belt can take place.

The reinforcing cables in the belt normally do not cause a net signal toappear at the input of band pass filter 10, because their positionrelative to the pickup coils changes only slowly; such slow signals arecancelled by the self-balancing circuit previously described. However,at splices and patches in the belt, these reinforcing cables are spacedat double density. When a patch or reinforced splice passes the coils,there is an abrupt rise in the signal voltage output of the pickup loopsas the leading edge passes under the loops, followed by an abrupt dropin signal voltage as the trailing edge passes, due to the increase inthe amount of metal in the spliced or patched area; this metal is ofcourse moving at belt speed. It is necessary to provide a splicecompensating arrangement, in order to prevent an alarm being sounded inresponse to the passage of these patches and splices in the belt.

A belt splice pickup assembly 86, which functions in a manner similar tothe magneto of a small engine, is placed under the conveyor belt 77.This assembly comprises two series-connected' coils of wire 78 wound onthe legs of a U- shaped soft iron core 79, with a permanent magnet 80supplying a magnetic flux through the core. The steel reinforcing cablesin the belt 77 complete the magnetic loop. With the belt 77 running, thecontinuous cables therein cause a substantially steady magnetic flux inthe core 79, resulting in no induced voltage in the coils 78, When asplice 81 approaches the coil structure, there is twice as much metalcompleting the magnetic coupling of the magnet to the core, causing anincrease in the magnetic flux through the core, thus inducing a voltagein the pickup coils 78. When the splice passes the coil structure, themagnetic coupling decreases to normal, causing a change of magnetic fluxin the opposite direction and inducing in the coils 78 a voltage of theopposite polarity.

The voltage pulse produced in coils 78 by the leading edge of the splice81 (resulting from the change in flux through gering pulse to amonostable (one-shot) multivibrator 84 which has an adjustable transfertime. The multivibrator output is inverted by a phase inverter 85(typically, a solid-state device) and then applied as a bias throughresistor 68 to the SCR 63 in alarm circuit 57. The time interval duringwhich this bias is so applied depends upon the (adjustable) transfertime of multivibrator 84, and is adjusted to be slightly longer than thetime required for the longest splice (the splices and patches being ofknown lengths) to pass the pickup loops and 6.

The SCR biasing circuit described is designed to produce a voltage onthe SCR 63 equal to that produced (at the output of amplifier 56) by'themetal detector pickup loops 5 and 6 when the splice 81 passes them.Thus, when a splice arrives at the pickup loops 5 and 6, a bias isapplied to the SCR 63 sufficient to prevent it from being fired (tothereby drop out the alarm relay 65), unless there is simultaneouslypresent a large signal from a piece of tramp metal being carried by thebelt 77.

As previously stated, the bias timer (the transfer time of multivibrator84, which is adjustable) is set to permit the iongest splice to pass themetal detector pickup loops 5 and 6. Normal operation is resumed afterthe timed interval, by the multivibrator 84 reverting to its originalcondition, thereby to remove the bias applied to the SCR 63. The metaldetecting circuit is not inoperative during the timed interval (passageof splice), but its sensitivity is somewhat reduced.

Since the belt splice compensating arrangement 87 (which functions as abelt splice detector and biasing circuit, producing a bias on the SCR63) must operate before the metal detector can sound an alarm, thepickup assembly 86 thereof is positioned slightly ahead of the metaldetector pickup loops 5 and 6, with reference to the direction of travelof the belt 77.

We claim:

1. Apparatus for detecting the fortuitous presence of a metallic bodytraveling at a known rate through a detection zone, comprising anexcitation coil for setting up an alternating magnetic field in saidzone, means for supplying an alternating excitation current to saidcoil, a pickup coil means in said zone, the entry of a metallic bodyinto said field resulting in the setting up of eddy currents in suchbody and the consequent inducing ofa signal voltage in said pickup coilmeans; means for establishing an alternating bucking voltagecontrollable in phase and amplitude and of the same frequency as saidexcitation current, means for algebraically adding said bucking voltageto the voltage induced in said pickup coil means to produce a resultantvoltage, means responsive to said resultant voltage for automaticallycontrolling the phase and amplitude. of said bucking voltage to maintainsaid resultant voltage at a minimum, said last-mentioned means having atime constant in excess of the time required for a metallic body, movingat said rate, to travel through said zone; and an indicating circuitresponsive to said resultant voltage.

2. Apparatus in accordance with claim 1, wherein said pickup coil meanscomprises a pair of coils connected in series-aiding relationship andpositioned in overlapping relation with respect to said excitation coilbut on respective opposite sides of the critical coupling position withrespect thereto.

3. Apparatusin accordance with claim 1, wherein said alternating buckingvoltage establishing means comprises means coupled to said alternatingexcitation current supplying means for deriving from such current fourvoltages in phase quadrature, and separate voltage-controllable meansfor determining the amplitude of each of said quadrature voltages fed toa summing means to provide said bucking voltage.

4. Apparatus as defined in claim 3, wherein said phase and amplitudecontrolling means comprises means responsive to said resultant voltagefor producing a pair of control voltages whose polarities and magnitudesdepend upon the relative phase and magnitude of said resultant voltage,and means for applying said control voltages to saidvoltage-controllable means.

5. Apparatus as defined in claim 3, wherein said phase andamplitude'controlling means comprises a pair of synchronous rectifiersfor rectifying said resultant voltage, means for driving said rectifiersrespectively by two voltages of the same frequency as said excitationcurrent and in phase quadrature with respect to each other, and meansfor applying the output voltages of said rectifiers as control voltagesto said voltagecontrollable means.

6. Apparatus according to claim 5, wherein said time constant isprovided by series resistors and shunt capacitors connected in thecircuits of said synchronous rectifiers.

7. An apparatus for detecting the fortuitous presence of a metallic bodymoving along a metallically-reinforced conveyor belt known-lengthportions of which are nonuniform as to the amount of metal containedtherein, comprising an excitation coil for setting up an alternatingmagnetic field in a zone located along the length of said belt, meansfor supplying an alternating excitation current to said coil, a pickupcoil means in said zone, the entry of a metallic nonuniformity into saidfield resulting in the inducing of a signal voltage in said pickup coilmeans; an indicating circuit of controllable sensitivity responsive to avoltage induced in said pickup coil means, means positioned adjacentsaid zone for detecting the approach of a metallic nonuniformity to saidzone and for producing a voltage in response thereto, and means forutilizing said last-mentioned voltage to reduce the sensitivity of saidindicating circuit while still permitting fortuitously-present metallicbodies to be indicated by said circuit.

8. Apparatus in accordance with claim 7, wherein said lastmentionedmeans operates to reduce the sensitivity of said indicating circuit to adegree commensurate with the amplitude of said signal voltage, therebyto prevent operation of said indicating means in response to said signalvoltage.

9. Apparatus in accordance with claim 7, wherein said lastmentionedmeans has incorporated therein a time delay such as to maintain thesensitivity of said indicating circuit reduced for a time intervalslightly in excess of the time required for the longest metallicnonuniformity to pass said pickup coil means.

10. Apparatus in accordance with claim 7, wherein said detecting meansis positioned ahead of said pickup coil means, with reference to thedirection of travel of said belt.

11. Apparatus for detecting the fortuitous presence of a metallic bodymoving at a known rate along a metallicallyreinforced conveyor beltknown-length portions of which are nonuniform as to the amount of metalcontained therein, comprising an excitation coil for setting up analternating magnetic field in a zone located along the length of saidbelt, means for supplying an alternating excitation current to saidcoil, a pickup coil means in said zone, the entry of any nonuniformmetallic body into said field resulting in the inducing of a signalvoltage in said pickup coil means; means for establishing an alternatingbucking voltage controllable in phase and amplitude and of the samefrequency as said excitation current, means for algebraically addingsaid bucking voltage to the voltage induced in said pickup coil means toproduce a resultant voltage; means responsive to said resultant voltagefor automatically controlling the phase and amplitude of said buckingvoltage to maintain said resultant voltage at a minimum, saidlast-mentioned means having a time constant in excess of the timerequired for a nonuniform metallic body, moving at said rate, to travelthrough said zone; an indicating circuit of controllable sensitivityresponsive to said resultant voltage, means positioned adjacent saidzone for detecting the approach of one of said metallic nonuniformitiesto said zone and for producing a voltage in response thereto, and meansfor utilizing said last-mentioned voltage to reduce the sensitivity ofsaid indicating circuit.

12. Apparatus in accordance with claim 11, wherein said pickup coilmeans comprises a pair of coils connected in series-aiding relationshipand positioned in overlapping relation with respect to said excitationcoil but on respective opposite sides of the critical coupling positionwith respect thereto.

due to the entry of one of said metallic nonuniformities into saidfield, thereby to prevent operation of said indicating means in responseto said last-mentioned signal voltage.

15. Apparatus in accordance with claim 1], wherein said last-mentionedmeans has incorporated therein a time delay such as to maintain thesensitivity of said indicating circuit reduced for a time intervalslightly in excess of the time required for the longest one of saidmetallic nonuniformities, moving at said rate, to pass said pickup coilmeans.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 573784 Dated April 6 1971 Inventor) Henry L. Bachofer et a1 It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

On the cover sheet insert [73] Assignee Great Canadian Oil SandsLimited, Toronto Canada, a corporation of Canada Signed and sealed this7th day of December 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer ActingCommissioner of Pate

1. Apparatus for detecting the fortuitous presence of a metallic bodytraveling at a known rate through a detection zone, comprising anexcitation coil for setting up an alternating magnetic field in saidzone, means for supplying an alternating excitation current to saidcoil, a pickup coil means in said zone, the entry of a metallic bodyinto said field resulting in the setting up of eddy currents in suchbody and the consequent inducing of a signal voltage in said pickup coilmeans; means for establishing an alternating bucking voltagecontrollable in phase and amplitude and of the same frequency as saidexcitation current, means for algebraically adding said bucking voltageto the voltage induced in said pickup coil means to produce a resultantvoltage, means responsive to said resultant voltage for automaticallycontrolling the phase and amplitude of said bucking voltage to maintainsaid resultant voltage at a minimum, said last-mentioned means having atime constant in excess of the time required for a metallic body, movingat said rate, to travel through said zone; and an indicating circuitresponsive to said resultant voltage.
 2. Apparatus in accordance withclaim 1, wherein said pickup coil means comprises a pair of coilsconnected in series-aiding relationship and positioned in overlappingrelation with respect to said excitation coil but on respective oppositesides of the critical coupling position with respect thereto. 3.Apparatus in accordance with claim 1, wherein said alternating buckingvoltage establishing means comprises means coupled to said alternatingexcitation current supplying means for deriving from such current fourvoltages in phase quadrature, and separate voltage-controllable meansfor determining the amplitude of each of said quadrature voltages fed toa summing means to provide said bucking voltage.
 4. Apparatus as definedin claim 3, wherein said phase and amplitude controlling means comprisesmeans responsive to said resultant voltage for producing a pair ofcontrol voltages whose polarities and magnitudes depend upon therelative phase and magnitude of said resultant voltage, and means forapplying said control voltages to said voltage-controllable means. 5.Apparatus as defined in claim 3, wherein said phase and amplitudecontrolling means comprises a pair of synchronous rectifiers forrectifying said rEsultant voltage, means for driving said rectifiersrespectively by two voltages of the same frequency as said excitationcurrent and in phase quadrature with respect to each other, and meansfor applying the output voltages of said rectifiers as control voltagesto said voltage-controllable means.
 6. Apparatus according to claim 5,wherein said time constant is provided by series resistors and shuntcapacitors connected in the circuits of said synchronous rectifiers. 7.An apparatus for detecting the fortuitous presence of a metallic bodymoving along a metallically-reinforced conveyor belt known-lengthportions of which are nonuniform as to the amount of metal containedtherein, comprising an excitation coil for setting up an alternatingmagnetic field in a zone located along the length of said belt, meansfor supplying an alternating excitation current to said coil, a pickupcoil means in said zone, the entry of a metallic nonuniformity into saidfield resulting in the inducing of a signal voltage in said pickup coilmeans; an indicating circuit of controllable sensitivity responsive to avoltage induced in said pickup coil means, means positioned adjacentsaid zone for detecting the approach of a metallic nonuniformity to saidzone and for producing a voltage in response thereto, and means forutilizing said last-mentioned voltage to reduce the sensitivity of saidindicating circuit while still permitting fortuitously-present metallicbodies to be indicated by said circuit.
 8. Apparatus in accordance withclaim 7, wherein said last-mentioned means operates to reduce thesensitivity of said indicating circuit to a degree commensurate with theamplitude of said signal voltage, thereby to prevent operation of saidindicating means in response to said signal voltage.
 9. Apparatus inaccordance with claim 7, wherein said last-mentioned means hasincorporated therein a time delay such as to maintain the sensitivity ofsaid indicating circuit reduced for a time interval slightly in excessof the time required for the longest metallic nonuniformity to pass saidpickup coil means.
 10. Apparatus in accordance with claim 7, whereinsaid detecting means is positioned ahead of said pickup coil means, withreference to the direction of travel of said belt.
 11. Apparatus fordetecting the fortuitous presence of a metallic body moving at a knownrate along a metallically-reinforced conveyor belt known-length portionsof which are nonuniform as to the amount of metal contained therein,comprising an excitation coil for setting up an alternating magneticfield in a zone located along the length of said belt, means forsupplying an alternating excitation current to said coil, a pickup coilmeans in said zone, the entry of any nonuniform metallic body into saidfield resulting in the inducing of a signal voltage in said pickup coilmeans; means for establishing an alternating bucking voltagecontrollable in phase and amplitude and of the same frequency as saidexcitation current, means for algebraically adding said bucking voltageto the voltage induced in said pickup coil means to produce a resultantvoltage; means responsive to said resultant voltage for automaticallycontrolling the phase and amplitude of said bucking voltage to maintainsaid resultant voltage at a minimum, said last-mentioned means having atime constant in excess of the time required for a nonuniform metallicbody, moving at said rate, to travel through said zone; an indicatingcircuit of controllable sensitivity responsive to said resultantvoltage, means positioned adjacent said zone for detecting the approachof one of said metallic nonuniformities to said zone and for producing avoltage in response thereto, and means for utilizing said last-mentionedvoltage to reduce the sensitivity of said indicating circuit. 12.Apparatus in accordance with claim 11, wherein said pickup coil meanscomprises a pair of coils connected in series-aiding relationship andpositioned in overlapping rElation with respect to said excitation coilbut on respective opposite sides of the critical coupling position withrespect thereto.
 13. Apparatus in accordance with claim 11, wherein saidalternating bucking voltage establishing means comprises means coupledto said alternating excitation current supplying means for deriving fromsuch current four voltages in phase quadrature, and separatevoltage-controllable means for determining the amplitude of each of saidquadrature voltages fed to a summing means to provide said buckingvoltage.
 14. Apparatus in accordance with claim 11, wherein saidlast-mentioned means operates to reduce the sensitivity of saidindicating circuit to a degree commensurate with the amplitude of asignal voltage induced in said pickup coil means due to the entry of oneof said metallic nonuniformities into said field, thereby to preventoperation of said indicating means in response to said last-mentionedsignal voltage.
 15. Apparatus in accordance with claim 11, wherein saidlast-mentioned means has incorporated therein a time delay such as tomaintain the sensitivity of said indicating circuit reduced for a timeinterval slightly in excess of the time required for the longest one ofsaid metallic nonuniformities, moving at said rate, to pass said pickupcoil means.